Hydrogen ion electrodes used for measuring the concentration of hydrogen ions in aqueous solutions and pH meters using a glass electrode are widely known. A "solid membrane-type" hydrogen ion electrode has also been proposed in which an ion sensing material such as dodecamolybdophosphoric acid (Kodaiku, Murata and Ikeda, Preprint for the 34th Meeting of Anal. Chem. Soc. Jap., 2D05, p. 489, 1985) or dodecylamine (D. Ammann et al., Anal. Chem., 53, 2267, 1981) is immobilized with a suitable material such as a polymer. There have also been proposed ion-selective field-effect transistors (ISFET) that use an oxide (e.g., SiO.sub.2, Al.sub.2 O.sub.3 or Ta.sub.2 O.sub.5), Si.sub.3 N.sub.4 (Matsuo and Esashi, Denki Kagaku, 50, 64, 1982), or titanium nitride (TiN) (Wakida, Makabe, Mochizuki, Yamane and Higashi, Preprint for the 58th Spring Meeting of Chem. Soc. Jap., 1,3IG17, p. 360, 1989) as a hydrogen ion sensing material.
In lead-acid batteries, the charge-discharge reaction takes place according to the following scheme: ##STR1##
As the discharge proceeds in the battery, sulfuric acid (H.sub.2 SO.sub.4) as the electrolyte is consumed and its concentration decreases. Conversely, if the charge proceeds, sulfuric acid is produced and its concentration increases. Thus, the consumption or production of sulfuric acid in the lead-acid battery is proportional to the amount of discharge or charge (ampere-hour), and thus a specific stage of the transition from a charged to a discharged state or vice versa, or residual capacity of the battery can be determined by measuring the sulfuric acid concentration.
A float-type hydrometer and a light refraction-type hydrometer are commercially used today to determine a specific stage of the transition from a charged to a discharged state or vice versa of lead-acid batteries using the principle described above. However, these hydrometers are generally bulky and are not used as an integral part of the lead-acid battery. Instead, part of the electrolyte is pumped out of the battery so that the specific gravity of the electrolyte can be measured with the hydrometers. Therefore, those hydrometers are not applicable to sealed lead-acid batteries.
To enable measuring the electrolyte concentration in sealed lead-acid batteries, various methods have been proposed. One method determines the concentration of an aqueous sulfuric acid solution by measuring humidity with a humidity sensor relying on the vapor pressure in the space above the sulfuric acid solution which is in gas-liquid equilibrium with the solution being dependent on its concentration, as described in West German Patent No. 2,254,207. This method has been refined such that the partial pressure of water vapor diffusing through pores in a porous polypropylene membrane which envelops a humidity sensor is measured with the sensor (J. L. Weininger et al., J. Electrochem. Soc., 129, 2409, 1982). Another method uses an electrode-type hydrometer that operates on the principle that the potential difference between a lead dioxide electrode and a lead electrode depends on the concentration of sulfuric acid (see, for example, Japanese Patent Unexamined Publication No. Sho. 60-62066).
Hydrogen ion electrodes typified by pH meters have heretofore been inapplicable in strong acidic or alkaline aqueous solutions since the linearity based on the Nernst equation generally cannot be obtained outside the so-called "pH measuring range" (pH 2-12), and this phenomenon is commonly referred to as "acid error" or "alkali error". The concentration of the aqueous sulfuric acid solution used in lead-acid batteries is generally within the range of ca. 5.2-0.8 mol/L (ca. 1.28-1.05 in terms of specific gravity), so there has been no reported attempt of using a hydrogen ion electrode as a sensor for measuring sulfuric acid concentration in lead-acid batteries.
The only exception is the solid membrane-type hydrogen ion sensor that has dodecamolybdophosphoric acid fixed with a binder (Kodaiku et al., Preprint for the 34th Meeting of Anal. Chem. Soc. Jap., ibid.). Although the report asserts that the operation of the sensor complies with the Nernst equation over the working concentration range of the aqueous sulfuric acid solution used in the lead-acid battery, this method is still in the development stage and lacks reliability.
A method of using an ion-selective field-effect transistor (ISFET) in a device for sensing the specific gravity of lead-acid batteries has already been made public (Japanese Patent Unexamined Publication No. Sho. 60-12266), but there is no disclosure as to what material the ion sensing layer in the ISFET contains. Tantalum oxide (Ta.sub.2 O.sub.5) has been reported to exhibit the best characteristics as the oxide and nitride coatings provided on the field-effect transistor to measure the hydrogen ion concentration (Matsuo and Esashi, Oyo Butsuri, 49, 586, 1980). However, the ISFET using Ta.sub.2 O.sub.5 is disadvantageous in that if the ISFET is submerged in sulfuric acid having a concentration commonly used for the electrolyte in lead-acid batteries, the potential of the sensor will become unstable in a few months.
Further, the method of measuring the sulfuric acid concentration with the conventional humidity sensor is generally slow in its response, and has the added disadvantage that the vapor of sulfuric acid will corrode the sensor. The electrode-type hydrometer described in Japanese Patent Unexamined Publication No. Sho. 60-62066 suffers from a serious operational disadvantage in that to compensate the self-discharge of the electrode, an electric current must occasionally be supplied from an external circuit to conduct electrochemical oxidation and reduction.
Sealed lead-acid batteries present another problem that is peculiar to their structure. Generally, a very small amount of an aqueous solution of sulfuric acid is retained in the glass mat which serves not only as a separator, but also as an electrolyte retainer. Thus, the presence of a free electrolyte is negligible. Hence, a device for sensing hydrogen ion concentration must be used which is compact enough to measure the concentration of sulfuric acid present in the glass mat.